How We Know Climate Change Affected Extremes in 2017

December 30, 2017

STANFORD, Calif. — This was a year of devastating weather, including historic hurricanes and wildfires here in the United States. Did climate change play a role? Increasingly, scientists are able to answer that question — and increasingly, the answer is yes.

Consider Hurricane Harvey, which caused enormous destruction along the Gulf Coast; it will cost an estimated $180 billion to recover from the hurricane’s storm surge, high winds and record-setting precipitation and flooding. Did global warming contribute to this disaster?

The word “contribute” is key. This doesn’t mean that without global warming, there wouldn’t have been a hurricane. Rather, the question is whether changes in the climate raised the odds of producing extreme conditions.

It is therefore critical to examine all of the contributing factors. In the case of Hurricane Harvey, these include the warm ocean that provided energy for the storm; the elevated sea level on top of which the storm surge occurred; the atmospheric pressure pattern that contributed to the storm’s stalling over the coast; and the atmospheric water vapor that provided moisture for the record-setting precipitation.

The first step is to ask whether historical changes have been observed in any of the factors. For example, ocean temperatures have increased in recent decades. Applying the same statistical techniques used in engineering, medicine and finance, we can analyze whether those increases have changed the odds of achieving this year’s warm temperatures in the tropical Atlantic and Gulf of Mexico.

But identifying a trend doesn’t tell us the cause. For that, we run controlled experiments using computerized climate models that simulate conditions in previous decades, with and without the variable of human-generated greenhouse gases in the atmosphere. By comparing those experiments with historical weather data, we can quantify how likely an event is with and without human-generated warming. Based on previous warm years, we can expect to find that human-generated warming influenced this year’s ocean temperatures.

We also know that global warming is increasing the moisture in the atmosphere, meaning that a given storm can produce more precipitation. Analyses by Kerry Emanuel of M.I.T. and others since the storm show that global warming makes heavy rainfall during storms like Hurricane Harvey more likely.

Further, Hurricane Harvey’s stalling over the coast was critical for the record rainfall. The exact meteorological causes are complex, but the pattern of atmospheric pressure across North America played an important role. We have found that global warming increased the odds of the pressure pattern that contributed to the 2010 Russian heat wave that killed more than 50,000 people. We can likewise look back at pressure patterns during past hurricane seasons and examine whether global warming has altered the odds of patterns similar to Hurricane Harvey’s.